Participants in this research study have an inherited condition that results in an elevated level of cholestanol (a break-down product of cholesterol) in the blood and tissues and a very reduced amount of a bile acid called chenodeoxycholic in the bile. The purpose of the study is to: 1) determine if feeding a diet that contains no cholestanol and no cholesterol will lower the amount of cholestanol in the blood 2) determine if taking simvastatin (Zocor), a drug that lowers the level of cholesterol in the blood and taking chenodeoxycholic acid will lower the level of cholestanol in the blood and reduce the size of the patient's xanthoma and make them feel better, 3) determine if and how much cholesterol and cholestanol are removed from the Achilles tendon after two years on medication and 4) use cells from a skin biopsy to study the genetics of CTX (cerebrotendinous xanthomatosis).